The Geomagnetic Cusps: Magnetic Topology and Physical Processes Antonius Otto Thanks to: Eric Adamson, Katariina Nykyri, Julia Pilchowski, Jason McDonald

The Geomagnetic Cusps Chapman and Ferraro, ~1930

The Geomagnetic Cusps Structure: Magnetic structure (topology) – Magnetic null point => B=0 Magnetic boundary maps to cusp point! Identification Topics: Structure, Morphology Location, size, formation Geomagnetic significance Plasma entry Precipitation Ion outflow Waves and Turbulence Particle trapping Physical processes Magnetic reconnection (Kelvin Helmholtz instability Turbulence) Diamagnetic Cavities and Cusp Energetic Particles Formation Particle source and acceleration Summary

The Geomagnetic Cusps Chapman and Ferraro (1931)

Role of the cusps in magnetospheric physics

The Geomagnetic Cusps - Structure Haerendel et al., 1978Zong et al., 2005 Structural Elements: Stagnation region Entry Layer (HLBL - High Latitude Boundary Layer) Midaltitude cusp LLBL (Low Latitude Boundary Layer) Plasma Mantle

Cusp Observations and Properties Exterior Cusp Stagnant flow Weak magnetic field High plasma  Plasma properties different from magnetosheath Thin boundaries Cluster Observations: Lavraud et al., 2002 V n T B 

The Geomagnetic Cusps - Structure Exterior Cusp: Region with strong magnetic field variation and depression! Lavraud et al. (2004)

The Geomagnetic Cusps - Structure Lavraud et al. (2004) Exterior cusp: Boundaries: Lobes, dayside plasma sheet, and magnetosheath Stagnant plasma (particularly for B z > 0) Total pressure balance

The Geomagnetic Cusps - Structure Newell et al. (2005) B z < -1nT B z > 1nT B y > 3nTB y < -3nT Cusp Precipitation: Low energy, high number density flux (e.g. Burch, 1968; Heikkila and Winningham, 1971; Frank, 1971; Newell et al., 2004 …) Ion precipitation (Proton aurora;

The Geomagnetic Cusps - Boundaries Zhang et al. (2006), also Dunlop (2005), Nykyri (2010)

The Geomagnetic Cusps – Precipitation + Ion outflow Fuselier et al. (2003)Fuselier et al. (2008) Ion outflow distribution Cusp aurora for changing IMF orientation Cusp aurora Also! Relation between upflow and poleward moving auroral transients (and intermittent reconnection) (e.g., Moen et al., 2004)

The Geomagnetic Cusps – Precipitation + Ion outflow Net hemispheric outflow during quiet times (Peterson et al., 2008) Ionosphere major plasma source for magnetosphere (e.g. Chappell et al, ‘87, Lockwood et al., ’85; Horwitz and Moore, ’97)

Expected Cusp Processes Local magnetic shear varies 360 degrees: Antiparallel magnetic field => Magnetic Reconnection Parallel and antiparallel field => Kelvin Helmholtz Fast (superfast) flow past an obstacle => Turbulence Low magnetic field strength Issue: Difficult in local models V sh B sh V sh

Cusp Reconnection (Crooker, '79; Song and Russell, '92;..) Dorelli et al, 2007 Importance of magnetic null points? Antiparallel vs. component reconnection? Sash (White et al., 1998)

Cusp Reconnection – Global simulation Li et al., '05

Cusp Reconnection Dayside Hybrid Simulations (Lin and Wang, ’06)

Cusp Reconnection - antiparallel vs component? Fuselier et al.

Cusp Reconnection Observations (Fuselier, Phan, Trattner, Wang, Lavraud,..) Ground based (lobe reconnection cells, particle signatures) In-situ spacecraft observations Remote particle signatures Trattner et al, 2004

Polar Magnetic Field and Particle Observations – 1 Fritz and Chen, 2001Fritz and Chen, 2003 (Also Chen et al., 1997, 1998; Sheldon et al., 1998;...) Diamagnetic cavities: High level of magnetic and density fluctuations Frequently associated with strongly enhanced fluxes of energetic particles

Magnetic Field and Particle Observations – 2 CEP’s - Cusp Energetic Particle Events: Regions of `turbulent’ weak magnetic field Enhanced energetic particle fluxes at/above 10 keV Particle energy proportional to charge Magnetic moment consistent with ring current/ radiation belt Nykyri et al., 2008 D’cavity M’sheath D’cavity M’sheath Cluster/RAPID Protons

Turbulence or structure Nykyri et al. (2010)

Sharp Transitions between Regions with and without CEP’s Regions with CEP’s map to quasi-parallel bow shock. Cusp Closed Field Lines Region Trattner et al. (2009)

Questions: Local Acceleration? Quasi-parallel bow shock source? Magnetospheric source (Aiskainen and Mursula, 2006)? What is the acceleration mechanism? Turbulence, Betatron, Fermi, Potential, Other?

Presence of energetic electron in cavities. Only small fraction of distribution in the loss cone (to the magetosheath) Walsh et al. (2007) Nykyri et al. (2010) Cluster/RAPID Electrons ISEE 1

IMF-connected Magnetospheric field lines Initial Configuration Local MHD + Test Particle Simulations

3D Cusp-like Reconnection

Typical Properties Enhanced Density and Pressure Thin Current sheet bounding exterior High Plasma Beta Flows into cusp from reconnection site

Diamagnetic Cavities Regions of strongly depressed magnetic field Scale: 4 to 6 R E parallel to boundary; 1 to 2 R E perpendicular to boundary Enhanced pressure and density duskdawn dusk dawn

Cavity locations superposed for 3 IMF orientations. Cavities increase in size due to reconnection in region with small guide field.

Test Particle Dynamics –Model: Typical particle properties:

Particle Dynamics: Initial conditions - Shell distributions in velocity (e=500eV) - Random distribution in space - Color codes max energy (see next slide) - Number of particles: here x x y z

Particle Dynamics: Total/average energy perpend. total parall contrib. of energetic part. eV E┴E┴ E║E║ E time eV

Particle Dynamics: Evolution inside of simulation domain x x y z Pitch angle 90

Polar Observations: Pitchangle distribution Chen and Fritz, 2004

Simulation: Particle Fluxes Triangles – flux constructed from test particles in the simulation domain (left) and particles leaving the domain (right) Dashed – flux corresponding to a Maxwellian with the initial thermal energy Solid – flux corresponding to a Maxwellian with the maximum thermal energy flux Energy Protons He ++

Particle Dynamics: Example 1 30keV

Particle Dynamics: Example 1

x y ‘Potential’ isosurfaces Particle motion in Cusp ‘potential’

Scaling of the acceleration process: Drift velocity: with Electric field: Energy: Mechanism: Highly efficient particle trapping. Particle motion: Combination of gradient curvature and ExB drift. Gradient curvature drift along the electric field component

Particle Dynamics: Scaling laws B 0 = 80 nT ~50 keV Particle drift paths: Particle Distrib: B 0 = 40 nT, L = 1 R E L = 2 R E Max. Energy: ~200 keV ~100 keV

Model: Energy gain for duskward ion motion and B z < 0 and dawnward ion motion and Bz > 0 Consistent with Cavity E y in Cluster event!

Summary on Particle Acceleration Mechanism: Highly efficient particle trapping. Particle motion: Combination of gradient curvature and ExB drift. Gradient curvature drift along the electric field component Scaling: Nonadiabaticity -> less spatially confined distributions, no contrib. to acceleration! Energy gain scales proportional to electric field ~E ~B 2 and length scale ~L 0 Temporal scale ~ 1/B, ~ L 0, and ~ 1/m Other: Energization not confined to inertial length scales! Primary energization in perpendicular direction. Parallel electric field = 0! Particle trapping + perpendicular electric field natural for high beta regions (magnetic neutral points, diamagnetic cavities,..) Solar particle acceleration Particle dynamics and acceleration of key importance in many space plasma systems

Concluding remarks: Cusps are rich in physics and play an important role for the magnetosphere Important aspects: Magnetic reconnection Particle entry and precipitation Ion outflow Generation or storage of energetic particles Unresolved (but progress): Origin of CEP’s Pitch angle distribution Fluxes are higher then in adjacent magnetosheath Presence of Oxygen Presence of energetic electrons